Generalized Anxiety Disorder (GAD): This is the most likely diagnosis for anxiety lasting less than six months due to its prevalence and the broad range of symptoms that can fit the criteria for GAD, including excessive worry about everyday things that is difficult to control.

Panic Disorder: Characterized by recurrent panic attacks, this condition can present with significant anxiety and may not always be immediately distinguishable from GAD, especially if the panic attacks are not the primary complaint.
Social Anxiety Disorder: Anxiety related to social situations where the person is exposed to possible scrutiny by others. This could be a likely diagnosis if the anxiety is primarily social in nature.
Adjustment Disorder with Anxiety: This diagnosis is considered when anxiety symptoms develop within three months of a significant life event or stressor and do not meet the criteria for another disorder.

Thyroid Disorders (Hyperthyroidism): Although less common, thyroid disorders can cause significant anxiety symptoms. Missing this diagnosis could lead to unnecessary suffering and potential complications if left untreated.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of anxiety, palpitations, and hypertension. It is crucial to rule out this condition due to its potential severity and the need for surgical intervention.
Substance-Induced Anxiety Disorder: Anxiety caused by the use or withdrawal of substances. This is a critical diagnosis not to miss, as treatment of the underlying substance issue can resolve the anxiety symptoms.

Anxiety Disorder Due to Another Medical Condition: Various medical conditions (e.g., heart disease, chronic obstructive pulmonary disease) can cause anxiety symptoms. While not as common, considering these conditions is essential for comprehensive diagnosis.
Delusional Disorder: Although primarily characterized by delusions, some forms can present with significant anxiety. This diagnosis is less likely but should be considered, especially if there are psychotic symptoms.
Post-Concussive Syndrome: Following a head injury, some individuals may experience anxiety among other symptoms. This is a rare consideration but important in the context of recent trauma.